A computer's registry accumulates a lot of corrupted, unused,
and unnecessary registry keys, especially if keys are not removed
when you uninstall a program. As a result, you may begin to
experience problems and your machine might function slower than
expected.

The latest version of Microsoft's RegClean could delete a lot
of such registry junk. You can download this from ftp://ftp.microsoft.com/Softlib/MSLFILES/REGCLEAN.EXE.